The present invention relates to an air treatment plant of the type including at least one waste gas vent connected to means for attenuating the noise conducted by this waste gas.
The invention applies especially to plants for producing a gas from air, typically oxygen by adsorption from air, particularly of the VSA (vacuum swing adsorption or vacuum-type pressure swing adsorption).
Such plants generally comprise at least one adsorption container and an air compressor connected by its delivery side to a first side (inlet) of the adsorption container via a feed line. Connected to the second side (outlet) of the adsorption container are a production line leading to the user site and an elution/repressurization line. A vacuum pump, generally a vacuum pump of the wet or dry Roots type, is connected to the first side of the adsorption container. All the components may be individually connected to the adsorption container and isolated therefrom by valves. The operation of such plants comprises a cycle with essentially three phases:
during the active or production phase, which generally takes place in the region of atmospheric pressure, the compressor draws in air from the environment and introduces it into the adsorber, where a first part of the constituents of air, essentially nitrogen, is adsorbed and a second part, especially oxygen, is recovered at the outlet of the adsorber as useful gas and then passes into the reservoir;
during a second phase or regeneration phase, the introduction of air into the adsorber is stopped. Thereafter, the adsorber is emptied of the nitrogen, firstly by applying a vacuum using the vacuum pump and secondly by purging at low pressure by means of the oxygen produced during the active phase (elution). The vacuum pump draws gas into the adsorber and discharges it via a silencer into the atmosphere;
during a third phase or repressurization phase, the adsorber is repressurized with oxygen.
In order to reduce the sound level of the gas delivered by a vacuum pump of a VSA plant, use is made, in the prior art, of a silencer in the air. Such a silencer generally comprises a sheet-metal shell having several parts. Reactive-type noise attenuation means (for example impedance tubes) and absorptive-type noise attenuation means (for example baffles) are provided, in succession, in this order, from the inlet to the outlet inside the various parts. Active-type noise attenuation means (for example, driven loudspeakers producing antinoise) may supplement or at least partly replace the reactive or absorptive systems.
In general, these silencers do not make it possible to achieve sufficiently low noise levels, especially when there are Roots-type vacuum pumps used which have a frequency spectrum difficult to attenuate because of a high proportion of low frequencies. In addition, the silencers are very bulky and their sheet-metal body radiates noise during operation. Moreover, such silencers are very expensive.
There are a great number of variants of plants for separating gases by pressure swing adsorption.
Two (non-limiting) examples of such plants are described in documents U.S. Pat. No. 5,223,004 and EP 0 598 321.
The object of the invention is to overcome the drawbacks explained above and to provide a silencer for an industrial plant of the aforementioned type which provides increased noise attenuation, is reduced in cost and is not very bulky.
For this purpose, the subject of the invention is a plant of the aforementioned type, characterized in that the noise attenuation means comprise a substantially closed and at least partially buried enclosure.
Depending on the particular embodiments, the invention may include one or more of the following characteristics:
the enclosure is completely buried;
at least part of the plant is placed above the said enclosure;
the plant includes at least one other vent connected to the said noise attenuation means;
the said noise attenuation means include, in succession, from a gas inlet to a gas outlet, reactive noise attenuation means and absorptive noise attenuation means;
the said reactive noise attenuation means comprise two reactive noise attenuation members in series;
the reactive noise attenuation means comprise impedance tubes;
the plant includes at least one vacuum pump, especially intended to deliver the waste gas, the delivery side of which is connected to the said noise attenuation means;
the plant includes means for connecting the suction side of the vacuum pump to the said noise attenuation means, for operation of the vacuum pump on itself;
the plant includes means for connecting the suction side of the vacuum pump to the said noise attenuation means downstream of the first reactive noise attenuation member and upstream of the second reactive noise attenuation member, while the delivery side of the vacuum pump is connected to the said reactive noise attenuation means upstream of the first reactive noise attenuation member;
the vacuum pump is a wet or dry Roots vacuum pump;
the enclosure is manufactured from concrete, masonry or the like;
the enclosure includes a gas exhaust duct, especially made of concrete, masonry or the like;
the duct includes additional noise attenuation means, especially of the absorptive or active type;
the enclosure includes water collection and extraction means;
the plant is an industrial plant for separating gases from air by pressure swing adsorption, typically for the production of oxygen from air; and
the enclosure comprises heat recovery means in order to bring the waste gas into heat exchange relationship with the incoming air.